It is common to make products having a uniform cross-section by extrusion from an aperture through which the raw material of the product is squeezed. In the case of plastic products, typically the plastic material is heated and forced under pressure to proceed through a shaping chamber, or mold portion, having an aperture through which the product is extruded. The plastic solidifies during or after its extrusion from the mold. It is then cooled and can be further processed by machining or cutting into required lengths.
There are many products that are elongated and would be suitable for formation by extrusion molding. However the extrusion is more difficult where a tapered product is planned, such as a fishing rod, a street lamp pole, a construction piling, or an antennae. The taper can be desired merely to save material near the top, where less structural strength is required, such as a street lamp pole, or the taper can planned for enhanced functionality, such as with a fishing pole, where initial flexibility near the tip is desirable, with increasing rigidity toward the base of the pole.
If the product to be formed is to be solid rather than hollow, it is possible to form it with extrusion by having an adjustable aperture such as an iris of overlapping blades that gradually close. However such apertures have been problematic for extrusion molding with plastic that is under pressure. The pressure can cause the plastic to enter between and jam or distort the seams between the jaws or among the iris blades.
There are additional problems if a hollow extruded product is desired. Having a complementary spindle or die set in the middle of an aperture in the mold in order to attempt hollowed extrusion is vulnerable to pressure because the spindle or die is difficult to support in the correct position. It cannot be supported right at the aperture without breaking the intended hollow. And it could not be made adjustable by having a fanning set of overlapping blades complementary to an outer iris's mechanism, because that would be even more subject to jamming or distorting failure than an outer iris. An outer iris' components could at least be supported from their immediate periphery, but having an inner die with fanning blades adjustable at ninety degrees to the extrusion flow could not be supported directly at the aperture without breaking the intended hollow in the formation of the product. A combination of extrusion and blow molding is suitable for short hollow objects where wall thickness tolerances fit within the blow deformation, expansion, compression, and strength parameters of the extruded material, but blow molding is not suitable for very elongated products such as street lamp poles. The temperature differential from one end of the product to the other during such a blow process typically overlaps the set temperature within a few feet, and extended control of the shape is only possible in any event for such elongated structures by having a post-extrusion outer (and inner if applicable) mold to control the blow, in which case the special problems of poured material molds arise again.
Clamping outside of and adjacent to an extrusion aperture can be used for shaping before a product is cooled. This method is used, for example, in producing corrugated tubing. However the method is a complication requiring both extra parts, energy and time. The shaping occurs outside the extrusion mold itself, necessitating a greater initial plastics temperature to allow pliability outside the mold itself. The process typically slows down what would otherwise be the maximum rate of extrusion for the material and product desired. Moreover, such post-mold processing is ill-suited to shaping the inside of the extruded product, as it is the outside of the extruded product that presents itself upon exit from the mold.
Elongated corrugated tube can be made in a traveling mold tunnel equipped with variegated shaped mold blocks. The seams between the mold blocks typically coincide with changes in the diameter of the tubing that forms the corrugations. The method is less suited where a smooth walled tube is desired, and is problematic where varying taper, whether external taper, internal taper, or both, is involved. A smooth wall, particularly on the outside is often desired in order to present a clean appearance, with a lack of irregularities that can become depositories for grime or dirt. Seams between rough patches on a wall can become cracks or shear lines that affect the rigidity and tensile strength of the product. Generally traveling mold tunnels have found to be unsuitable for producing smooth walled tubing due to the difficulties in controlling shear between an inner forming mandrel and an outer mold tunnel, quite apart from the additional difficulties of forming taper whether outer dimensional or relating to wall thickness.
It was common in the past for street lamp poles and telephone poles to be made from tree trunks. However even if the wood is pressure-treated with preservatives, the poles deteriorate in the ground, at their bases, and wherever exposed to the weather. Moreover such poles are not hollow, unless special work has been done on them, requiring external wiring. Even when new, treated wooden poles are often considered to be unsightly compared to uniform manufactured poles. Metal street lamp poles have been molded by pouring molten metal into static molds for hundreds of years. More recently, plastics technology has allowed the manufacture and use of high density, high strength polyethylene street poles, which are sufficiently strong to perform instead of wooden or metal poles and are less subject to rot or oxidization in the weather and in the ground. The problems for metal and wood poles can be even more severe in the case where they are used underwater, such as for pier pilings. High density polyethylene extrudable material has been found to be of sufficient compressive and tensile strength to replace wooden or metal poles in many such applications.
In the case of large, long poles such as street lamp poles, large amounts of material can be saved by having the poles tapered as well as hollow. The tapering is ideally not only an external taper but also an internal taper whereby the wall thickness is greater at the bottom of the pole and lesser near the top. The hollow is also functional for allowing wiring to be inserted up the pole.
If the poles, whether metal or plastic, are formed by pouring into a static mold in accordance with the prior technology, the mold needs to be unduly long and unwieldy to operate. Industrial floor space is wasted in keeping and using such molds, and energy is wasted in opening and closing such enormous molds. It is possible to reduce mold space by having a tunnel mold for forming long pipes, but such molds have not been suited for the manufacture of tapered poles.